1. Field of the Invention
The present invention relates to a non-contacting revolving speed detecting apparatus for detecting the speed of rotation of a rotary body by use of a laser and reflected and diffracted light therefrom.
2. Description of the Prior Art
Detection of the revolving speed of a rotary body has been required in various fields, and revolving speed detecting apparatus is often an indispensable element in the implementation of control techniques. Accordingly, simple but highly accurate revolving speed detecting devices have been required.
Conventional devices for detecting revolving speed of a rotary body can be roughly grouped into two types, i.e., contact and non-contact. The contact-type revolving speed detecting device includes the dynamo-type that uses electromagnetic induction in which an induced voltage is monitored to detect the revolving speed of a rotary body on the basis of the relation between the revolving speed and the induced voltage and the disc-type that uses an encoder plate having small inertia torque which is attached to a rotary shaft so as to detect magnetically or optically the revolving speed of a rotary body from the state of the encoder plate.
There are several types of non-contacting revolving speed detecting devices. One type uses a reflector attached to a rotary body and irradiated by a light source provided separately from the reflector so that the intensity of light reflected from the reflector is detected by a photo detector. A second type uses a magnetic pickup attached in the vicinity of a rotary body. A signal generated from the pickup when the rotary body approaches the pickup is magnetically detected to detect the revolving speed of the rotary body.
All the foregoing detecting means have certain disadvantages. In the contact-type revolving speed devices, the revolving speed detecting device is directly attached to a rotatable shaft of a rotary body with the consequence that the attachment of the device affects the rotation of the rotary body and makes it more difficult to detect the revolving speed of the rotary body accurately. In the non-contacting revolving speed detecting apparatus, on the other hand, the positional relation among a light source, a reflector, and a light detector must be strictly determined. This gives rise to severe design requirements. In the device using magnetic detection, high accuracy is required in the attachment of a magnetic pickup to a rotary body. Accordingly, it is difficult to produce such a device that is simple and also is highly accurate.